The present invention relates generally to the field of communications systems and more specifically to asymmetrical communication systems using a high data rate (wide data bandwidth) in one direction and a low data rate (narrow data bandwidth) for the return direction. The asymmetry lies in the relative data rates or amount of information flowing between two individual stations rather than a reference to the actual spectrum (bandwidth) of the transmissions. The principles of the present invention may however be extended to other communication environments including single direction and symmetrical two direction communication channels and to other fields requiring synchronization of remote communication equipment.
Systems in which relatively broadband information is transmitted to numerous users from a base and narrow band information from each user back to the base are known. For example, data is transmitted in an otherwise unused portion of a broadcast FM or TV signal and the users respond via dedicated telephone lines.
In the embodiments described below, time division (TD), particularly time division multiple access (TDMA), and spread spectrum (SS) transmission techniques are employed. Time division communication systems and spread spectrum transmission are known in the art, particularly in military and other secure communications systems. In a typical TDMA system, each user transmitter is provided with a spread spectrum receiver that monitors a synchronizing transmission from a base station. The synchronizing signal informs the user transmitter when to transmit so as not to interfere with the other transmitters in the system. Reception of such synchronizing transmissions adds considerable cost and complexity to conventional TDMA systems. Further background concerning time division communication systems can be found in Chapters 15 and 16 of Taub & Schilling, Principles of Communication Systems (2nd Ed., 1986).
The introductory paragraphs on spread spectrum modulation in Chapter 17 of Taub & Schilling describes the technique and some of its characteristics as follows:
"Spread spectrum is a technique whereby an already modulated signal is modulated a second time in such a way as to produce a waveform which interferes in a barely noticeable way with any other signal operating in the same frequency band. Thus, a receiver [A] tuned to receive a specific AM or FM broadcast would probably not notice the presence of a spread spectrum signal operating over the same frequency band. Similarly, the receiver [B] of the spread spectrum signal would not notice the presence of the AM or FM signal. Thus, we say that interfering signals are transparent to spread spectrum signals and spread spectrum signals are transparent to interfering signals.
To provide the `transparency` described above the spread spectrum technique is to modulate an already modulated waveform, either using amplitude modulation or wideband frequency modulation, so as to produce a very wideband signal. For example, an ordinary AM signal utilizes a bandwidth of 10 kHz. Consider that a spread spectrum signal is operating at the same carrier frequency as the AM signal and has the same power P, as the AM signal but a bandwidth of 1 MHz. Then, in the 10 kHz bandwidth of the AM signal, the power of the second signal is P.sub.s .times.(10.sup.4 /10.sup.6)=P.sub.s /100. Since the AM signal has a power P.sub.s, the interfering spread spectrum signal provides noise which is 20 dB below the AM signal."
Further background concerning spread spectrum techniques can be found in Chapter 17 of Taub & Schilling.